A table saw is a power tool used to cut a work piece to a desired size or shape. A table saw includes a work surface or table and a circular blade extends up through the table. A person uses a table saw by placing a work piece on the table and feeding it into contact with the spinning blade to cut the work piece to a desired size. The table saw is one of the most basic machines used in woodworking.
The blade in a table saw extends from below the work surface to an elevation above the work surface. Typically there is an opening in the table called a “throat” or “blade opening” through which the blade extends, and the blade opening is large enough for a user to perform some types of service or maintenance to the saw through the opening, such as changing the blade. An opening in the table around the blade, however, means the table cannot support a work piece next to the blade. Accordingly, an “insert” (also called a “throat plate”) is placed in the opening around the blade to support a work piece adjacent the blade. The insert includes a slot or channel through which the blade extends. The insert is removable so a user can access internal components of the saw through the blade opening (for example, a user can remove the insert to change the blade or to change a component in an active injury mitigation system such as a brake cartridge).
An insert may be very close to the blade, especially if the insert is a zero-clearance insert. A zero-clearance insert starts as a solid insert without any slot for the blade. The insert is placed in the blade opening in the saw table and the spinning blade is then raised through the insert to cut a slot. Using the blade to cut a slot in the insert insures that the insert remains as close as possible to the blade, or in other words, the insert has “zero clearance” to the blade.
Inserts typically lock in place in the blade opening so that the blade does not kick the insert back toward the user if the blade contacts the insert. Locking the insert in place also prevents the insert from popping up and presenting an edge that might catch a work piece as the work piece moves toward the blade. Also, a zero-clearance insert locks in place so that the insert is held down when the spinning blade rises up through the insert to cut the slot in the insert.
Some saws include active injury mitigation systems to detect when a dangerous condition occurs while using a saw and to react to minimize any injury. For example, some active injury systems detect when a user contacts the spinning blade and then stops and/or retracts the blade so quickly that a user typically receives only a small nick rather than a devastating injury. In these saws, an electrical signal is imparted to the blade and the active injury mitigation system monitors the signal for changes indicative of contact. In such a saw, an insert or throat plate typically is made of non-conductive material or coated with non-conductive material so that the electrical signal on the blade remains essentially unaffected if the blade contacts the insert. If the insert were made of conductive material and the blade contacted the conductive material, the electrical signal could be grounded and the active injury mitigation system could trigger or no longer be able to detect changes in the signal indicative of contact. Additionally, inserts made of conductive material can create a parasitic capacitance to the blade which can affect active injury mitigation systems.
Inserts need to be rigid and flat to provide support for work pieces and it has proven difficult to design inserts that are sufficiently rigid, flat and non-conductive. Typically, inserts used with active injury mitigation systems have been made of phenolic or wood to provide the required rigidity, flatness and non-conductivity.